1. Field of the Invention
The present invention relates to a wireless communication system. In particular, the present invention relates to a method of allocating pilot subcarriers in the wireless communication system including a multiple-input multiple-output (MIMO) antenna system.
2. Description of the Related Art
The Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard provides a technology of supporting a broadband wireless access and a protocol. The standardization has been progressed since 1999 and IEEE 802.16-2001 was approved in 2001. This has been established based on a single carrier physical layer called “WirelessMAN-SC”. In the IEEE 802.16a which was approved in 2003, “WirelessMAN-OFDM” and “WirelessMAN-OFDMA” were added to a physical layer in addition to “WirelessMAN-SC”. After the IEEE 802.16a standard was completed, a revised IEEE 802.16-2004 was approved in 2004. In order to correct bugs and errors of the IEEE 802.16-2004, IEEE 802.16-2004/Cor1 was completed in the form of “corrigendum” in 2005.
A MIMO antenna technology improves data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas. The MIMO technology was introduced in the IEEE 802.16a standard and has been continuously updated.
The MIMO technology is divided into a spatial multiplexing method and a spatial diversity method. In the spatial multiplexing method, since different data is simultaneously transmitted, data can be transmitted at a high speed without increasing the bandwidth of a system. In the spatial diversity method, since the same data is transmitted via multiple transmission antennas so as to obtain diversity gain, reliability of the data is increased.
A receiver needs to estimate a channel in order to restore data transmitted from a transmitter. Channel estimation indicates a process of compensating for distortion of a signal, which occurs by a rapid environment variation due to fading and restoring the transmission signal. In general, for channel estimation, the transmitter and the receiver need to know pilots.
In the MIMO system, a signal experiences a channel corresponding to each antenna. Accordingly, it is necessary to arrange the pilots in consideration of multiple antennas. While the number of pilots is increased as the number of antennas is increased, it is impossible to increase the number of antennas so as to increase a data transfer rate.
In the related art, different pilot allocation structures have been designed and used according to permutation (dispersion/AMC/PUSC/FUSC) methods. This is because the permutation methods are separated from each other along with time axis in the IEEE 802.16e system and thus structures could be differently optimized according to permutation methods. However, if the permutation methods coexist in certain time instance, a unified basic data allocation structure is necessary.
In the related art, since severe pilot overhead occurs, a transfer rate is decreased. In addition, since the same pilot structure is applied to adjacent cells or sectors, collision between the cells or the sectors may occur. Accordingly, there is a need for a method of efficiently allocating pilot subcarriers in the MIMO system.